Polyimide thermal isolation mesa for a thermal imaging system

ABSTRACT

A mesa (31) is formed from polyimide (or a similar polymer material) to achieve a high thermal resistance. In an exemplary thermal imaging application, an array of thermal isolation mesa structures (30) are disposed on an integrated circuit substrate (20) for electrically connecting and bonding a corresponding focal plane array (5) of thermal sensors (10). Each mesa structure (30) includes a polyimide mesa (31) over which is formed a metal conductor (32) that extends from the top of the mesa down a mesa sidewall to an adjacent IC contact pad (22). When the focal plane array (5) is bonded to the corresponding array of thermal isolation mesa structures (30), a thermally isolated, but electrically conductive path is provided between the sensor signal electrode (16) of the thermal sensor (10) and the corresponding contact pad (22) of the integrated circuit substrate (20).

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 07/387,918,filed Jul. 31, 1989, and entitled "POLYIMIDE THERMAL ISOLATION MESA FORA THERMAL IMAGING SYSTEM" now U.S. Pat. No. 5,047,644.

TECHNICAL FIELD OF THE INVENTION

This invention relates to thermal isolation structures for solid statesystems, and more particularly to a thermal (infrared) imaging systemusing a polyimide mesa structure and method for providing athermal-isolation intermediate structure to bond and electricallyconnect a focal plane array of ferroelectric thermal sensors to theunderlying integrated circuit substrate.

BACKGROUND OF THE INVENTION

One common application for thermal sensors is in thermal (infrared)imaging devices such as night vision equipment. One such class ofthermal imaging devices includes a focal plane array of ferroelectricthermal sensor elements coupled to an integrated circuit substrate witha corresponding array of contact pads. The thermal sensors define thepicture elements (or pixels) of the thermal image.

Each thermal sensor includes a ferroelectric (or pyroelectric) element,which may be a portion of a ferroelectric slab, formed from aferroelectric material that exhibits a state of electrical polarizationthat depends upon temperature (such as in response to thermalradiation). On opposing surfaces of the ferroelectric element aredisposed an infrared absorber electrode and a sensor signal electrode. Aferroelectric transducer element is formed by the infrared absorberelectrode and sensor signal electrodes, which constitute capacitiveplates, and the ferroelectric material, which constitutes a dielectric.

The thermal-image sensor signal appearing on the sensor signal electrodedepends upon the capacitance of the ferroelectric transducer element,which in turn depends upon incident infrared radiation (temperature).The sensor signals from the thermal sensors in the focal plane array arecoupled to an integrated circuit substrate which provides imageprocessing, with each thermal sensor (i.e., each sensor signalelectrode) being electrically coupled to a corresponding contact pad.

To maximize thermal response and ensure thermal image accuracy, eachferroelectric thermal sensor of the focal plane array must be thermallyisolated from the surrounding focal plane structure, and from theintegrated circuit substrate to insure that the associated transducercapacitance accurately represents incident infrared radiation.Thermal-isolation intermediate structures must be disposed between thefocal plane array and the integrated circuit substrate to provide abonding and sensor signal path interface that minimizes thermaldiffusion.

The intermediate thermal isolation structure comprises two elements--aconductor element and a thermal isolation element. This generalconfiguration for a thermal isolation structure can be represented by athermal circuit with two parallel thermal current paths, one through thelow-thermal-resistivity conductor and one through thehigh-thermal-resistivity thermal isolation structure. (See, for example,FIG. 2.) The design goal is to minimize the total thermal currentthrough these two paths.

Several approaches have been used to provide a thermal-isolationintermediate structure for isolating a thermal sensor array from anunderlying integrated circuit substrate. One approach is disclosed inU.S. Pat. No. 4,663,529 (Jenner), in which a square grid of channelsform a corresponding grid of pillars that define thermal sensorelements. Each pillar or sensor includes a central bore that is coatedwith a conductive layer. The conductive bores are dimensioned to be lessin diameter than corresponding electrode bumps disposed on an integratedcircuit substrate, such that when the focal plane array of pillars isdisposed over an integrated circuit substrate with a corresponding arrayof electrode bumps, the conductive bore of each thermal sensor rests on,and is electrically connected to, a corresponding electrode bump. Adisadvantage of this architecture is that mating the array ofconductive-bore pillars with the corresponding electrode bump arrayrequires close tolerances and exact alignment. Another disadvantage ofthis architecture is that photoresist is used as the structural materialfor the pillars, which are therefore structurally fragile andsusceptible to damage by solvents.

An alternative approach is disclosed in U.S. Pat. No. 4,143,269(McCormick), assigned to Texas Instruments Incorporated, the assignee ofthis invention, where a thermal-isolation intermediate structure for athermal sensor array uses conductive vias formed in a thermal isolationlayer (polyimide) that covers an integrated circuit substrate. In thisarchitecture, vias are formed in the thermal isolation layer, exposingcontact pads on the circuit substrate. The sensor signal electrode for athermal sensor is brought into contact with a corresponding conductivevia, providing an electrical connection to the associated contact pad. Adisadvantage of this architecture is that so much polyimide is presentthat total thermal resistance is relatively low. In addition, thisarchitecture requires a relatively large number of process steps,thereby increasing costs.

Heretofore, mesa structures have not been used to thermally isolate thethermal sensors in a focal plane array from an underlying integratedcircuit substrate. A mesa is a bump or pillar with a relatively smallcross-sectional area that projects from a substrate. Typically, mesastructures are formed by photolithographic techniques (either etch ordeposition processes). Mesa structures of materials other than polymermaterials (such as polyimides) have been used in the fabrication ofsolid state devices for such applications as providing arrays ofmultiprobe contacts or spacers.

Accordingly, a need exists for an improved thermal-isolationintermediate structure that provides a bonding and sensor signalinterface between a thermal sensor element and an underlying substrate.An advantageous structure would be capable of fabrication in arelatively few number of process steps.

SUMMARY OF THE INVENTION

The present invention improves thermal isolation between coupledcomponent structures of a solid state system (such as in a thermalimaging system with an array of thermal sensors bonded and electricallycoupled to an associated integrated circuit substrate), by providingthermally insulating mesa structures formed on and projecting from oneof the component structures.

In one aspect of the invention, the thermal isolation mesa structuresare used in a thermal imaging system to couple an array of thermalsensors to a circuit substrate that includes a corresponding array ofcontact pads. An array of mesa structures, each formed of a thermallyinsulating polymer material, project from the circuit substrate surfaceadjacent respective contact pads. Each mesa structure adjacent a contactpad includes a mesa conductor for providing a signal path from the topof the mesa to the contact pad. The thermal sensor array is disposedover the circuit substrate in contact with the mesa array such that, foreach thermal sensor, the sensor signal output is coupled through arespective mesa conductor to the associated contact pad.

In its more specific aspects, the thermal isolation mesa structures fora termal imaging system are formed from a polyimide material. Twoalternate configurations are recommended. In the first configuration,each polyimide mesa structure is formed with sloped sidewalls adjacent arespective contact pad of a circuit substrate, and a mesa-to-substratesignal path is provided by a mesa-strip conductor formed over the top ofthe mesa and down one sidewall, extending over the adjacent integratedcircuit contact pad. In the second configuration, the polyimide mesa isformed with substantially vertical sidewalls, and a mesa-to-substratesignal path is provided by a mesa-contour conductor formed over theentire mesa structure and extending onto the adjacent area of thecircuit substrate including over the adjacent contact pad.

The thermal imaging system includes a focal plane array of ferroelectricthermal sensors, each including a sensor signal electrode. The focalplane array is coupled to the circuit substrate by bump-bonding, with abump-bonding conductive material (such as a bump-bonding metal) beingprovided on the top of each mesa structure and/or on each sensor signalelectrode.

The polyimide mesa structure can be photolithographically fabricatedusing either photosensitive or non-photosensitive polyimide. Therecommended fabrication method is to use photosensitive polyimide,forming the mesa structures by patterning a layer of polyimide onphotoresist, and then developing the polyimide to remove the unexposedportions, leaving the polyimide mesa structures of the desiredconfiguration and array. After curing, the mesa-to-substrate conductorsare formed in conventional metal deposition procedures.

The technical advantages of the mesa structures of this inventioninclude the following. Forming the mesa structures from a polyimide (orother polymer) material provides a high thermal resistivity. Inaddition, a polyimide material is both compliant and resistant tosolvents. For example, in a thermal imaging system, using an array ofthermal isolation mesa structures provides an improved intermediatestructure for bonding and electrically connecting a focal plane array toan integrated circuit substrate. For such an application, the use ofpolyimide mesas provides a thermally insulated bonding/signal interface,as well as resistance to solvents and compliant compensation fortopological variations. The use of thermal isolation mesa structuresallows flexibility in design (including mesa configuration and signalpath configuration), and efficiency in fabrication.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and forfurther features and advantages, reference is now made to the followingDetailed Description, taken in conjunction with the accompanyingDrawings, in which:

FIG. 1 is an elevation view of a thermal imaging system showing aportion of a focal plane array of thermal sensor elements disposed above(prior to bonding) a corresponding portion of an integrated circuitsubstrate with a corresponding array of thermal isolation mesas andassociated integrated circuit contact pads;

FIG. 2 is a thermal circuit representation of a thermal isolationstructure;

FIGS. 3a and 3b are elevation and plan views of a portion of an array ofthermal isolation mesas with sloped sidewalls and mesa-strip conductorsfor electrically connecting the top of the mesa to an associated contactpad;

FIGS. 4a and 4b are elevation and plan views of a portion of an array ofthermal isolation mesas with vertical sidewalls and mesa-contourconductors for electrically connecting the top of the mesa to anassociated contact pad; and

FIG. 5 is an elevation view of a portion of an array of thermalisolation mesas that have been doped conductive to obviate the need fora conductor connection from the top of the mesa to the adjacent contactpad.

DETAILED DESCRIPTION OF THE INVENTION

The Detailed Description of the preferred embodiment of the thermalisolation mesa, and fabrication method, of the present invention isorganized as follows:

1. Thermal Isolation Mesa Structure

2. Mesa Structure (Sloped Sidewalls)

3. Mesa Structure (Vertical Sidewalls)

4. Fabrication

5. Additional Embodiments

6. Conclusion

The preferred embodiment of this invention is described in connectionwith an exemplary application for thermal imaging systems. The thermalisolation mesa structure is used to provide a thermally insulatedbonding and sensor-signal interface between a focal plane array ofthermal sensor elements (ferroelectrical transducers) and an integratedcircuit substrate used for image processing. The mesa structure of thisinvention is readily adaptable to other mesa structure applications.

1. Thermal Isolation Mesa Structure

As shown in FIG. 1, an exemplary thermal imaging system includes anuncooled focal plane array 5 of individual thermal sensor elements 10,together with an integrated circuit substrate 20 that includes on asubstantially planar surface a corresponding array of signal contactpads 22. An array of thermal isolation mesa structures 30, formed on theintegrated circuit substrate adjacent respective contact pads, providesa thermally insulated intermediate structure for bonding and sensorsignal connection.

Each thermal sensor 10 includes a ferroelectric element 12 formed from asuitable ferroelectric material, such as BST(barium-strontium-titanate). On opposing surfaces of ferroelectricelement 12 are disposed an infrared absorber electrode 14 and a sensorsignal electrode 16. The infrared absorber electrode is comprised of aninfrared absorber component and a metal electrode component (not shownseparately). Attached to the sensor signal electrode 16 is a metallicbump-bonding material 17 for providing a bump-bond to a correspondingthermal isolation mesa.

For each thermal sensor 10, the ferroelectric element 12, together withits infrared absorber electrode 14 and sensor signal electrode 16, forma ferroelectric transducer. That is, the metal electrode component ofthe infrared absorber electrode and the sensor signal electrodesconstitute capacitor plates, while the ferroelectric materialconstitutes a dielectric. The capacitors are temperature-dependent,implementing a ferroelectric (or pyroelectric) transducer function. Foreach thermal sensor electrode 10, thermal (infrared) radiation incidentto focal plane array 5 is absorbed by the infrared absorber componentand transmits as heat through the metal electrode component of theelectrode 14 into the ferroelectric material 12. The resultingtemperature change in the ferroelectric material causes a change in thestate of electrical polarization, i.e., a change in capacitance. Thecorresponding sensor signal output available from the sensor signalelectrode depends upon the temperature-dependent capacitance of theferroelectric transducer (i.e., the thermal sensor 10).

The integrated circuit substrate 20 comprises a conventional switchingmatrix and associated series of amplifiers. This integrated circuitsubstrate must be bonded to the focal plane array 5, with each contactpad 22 being electrically connected to the sensor signal electrode 16 ofa corresponding thermal sensor 10. The purpose of the thermal-isolationintermediate structure is to provide a bonding and sensor-signalinterface while maintaining a high degree of thermal isolation. Thermalisolation prevents the integrated circuit substrate 20 from acting as aheat sink for the thermal energy stored in the ferroelectric elements ofthe thermal sensors, adversely affecting transducer capacitance andsensor signal accuracy.

In accordance with this invention, the thermal isolation structure forbonding and electrically connecting focal plane array 5 to theintegrated circuit substrate 20 is provided by an array of thermalisolation mesa structures 30. A thermal isolation mesa structure 30includes a mesa formed from polyimide, an electrically and thermallyinsulating polymer material that exhibits a high degree of thermalresistance, and is highly resistant to solvents. In addition, thepolyimide is sufficiently compliant to allow for a certain amount oftopological variation, thereby reducing dimensional tolerancerequirements, without adversely impacting structural capability.

While polyimide is the recommended material for these mesas, otherorganic materials can be used. The primary design factors used inselecting an appropriate mesa material are thermal resistance andcompliance, as well as cost and ease of fabrication.

Each thermal isolation mesa structure 30 includes a polyimide mesa 31formed adjacent a contact pad 22. A metal conductor 32 provides a signalpath between the top of the mesa and the adjacent contact pad. Therecommended material for the mesa-to-substrate conductor is titaniumbecause of low thermal conductivity and ease of application.

A bump-bond metal 35 is formed onto the top of the thermal isolationmesa structure 30, i.e., over the top portion of the mesa-to-substrateconductor 32. Bump-bonding metal 35 is selected to be compatible withbump-bonding metal 17 on the sensor signal electrode 16. That is, for aconventional bump-bonding procedure, metals 17 and 35 are selected toprovide a good conductive bond between the signal sensor electrode 16 ofa thermal sensor 10, and the mesa-to-substrate conductor 32 of thecorresponding thermal isolation mesa structure 30, thereby providing agood signal connection between the thermal sensor and the associatedcontact pad.

The configurations of the polyimide mesa 31 and the associatedmesa-to-substrate conductor 32 are design choices, largely dependentupon thermal isolation and structural rigidity considerations. As shownin FIG. 2, a thermal isolation mesa structure 30 can be represented as athermal circuit connected between the sensor signal electrode 16(ignoring the bump-bonding metals 17 and 35), and the associated contactpad 22. A thermal current i_(T) flows through the thermal circuit 30(corresponding to the thermal isolation mesa structure) in two parallelthermal current paths--a conductor component i_(C) flows through amesa-to-substrate-conductor thermal resistance 32, while a mesacomponent i_(M) flows through a mesa thermal resistance 31. The designgoal is to maximize the total thermal resistance presented by themesa-to-substrate conductor 32 and the mesa 31, thereby minimizing thetotal thermal current i_(T) (i.e., i_(C) and i_(M))

The thermal current through a material depends upon the thermalconductivity of the material and the volume of material (thermalcapacitance). For the mesa structure 31, the polyimide exhibits very lowthermal conductivity (i.e., very high thermal resistivity). The size ofthe mesa will typically be dictated primarily by structural and thermalcapacitance considerations, so that the thermal-current mesa componenti_(M) through the mesa structure 31 will typically be dictated primarilyby structural requirements that determine the minimum allowable size ofthe mesa structure.

The mesa-to-substrate conductor 32 inevitably will exhibit a relativelylow thermal resistivity (whatever conductive material is selected), andtherefore a relatively high thermal conductivity. Accordingly, toincrease the total thermal resistance provided by the mesa-to-substrateconductor 32, and therefore minimize the corresponding thermal-currentconductor component i_(C), the conductor must be configured with assmall a cross sectional area to length aspect ratio as possible.

The recommended design approach is to first specify the structural andthermal capacitance requirements for the mesa 31, which will determineits total area and volume. Once the mesa configuration is selected, itsthermal resistance will be established, thereby establishing acorresponding thermal-current mesa component i_(m). Once this thermalcurrent component is established, a configuration for themesa-to-substrate conductor 32 is selected to achieve an overall thermalresistance that meets the thermal isolation requirements for a thermalsensor 10.

Two alternative configurations for mesa 31 are described below--a mesawith sloping sidewalls, and a mesa with vertical sidewalls. For thesloped-sidewall mesa, a mesa-strip configuration for the conductor 32 isrecommended (see Section 2), while for the vertical-sidewall mesa, amesa-contour configuration for the conductor is recommended (see Section3). These configurations are exemplary only, and other configurationsfor both the mesa structure and the mesa-to-substrate conductor areapparent to those skilled in the art. In particular, while both mesaconfigurations are shown as symmetrical in horizontal and vertical crosssection, such symmetry is not required.

Neither the focal plane array of thermal sensors, nor the integratedcircuit substrate form any part of the present invention. Likewise, themethod of bonding the focal plane array to the intermediatethermal-isolation mesa structure, including the selection ofbump-bonding, forms no part of the present invention. In particular,conventional bump-bonding techniques with bump-bonding metals such asindium and lead alloys may be used. Alternatively, a conductive epoxymay be substituted.

The thermal isolation mesa structure of this invention is readilyadaptable to thermal imaging systems in general to provide a thermallyinsulating bonding/conducting interface between a focal plane array ofthermal (ferroelectric) sensors and the associated integrated circuitsubstrate. Moreover, the mesa structure of the present invention hasgeneral applicability beyond thermal imaging devices (see Section 5).

2. Mesa Structure (Sloped Sidewalls)

FIGS. 3a and 3b show enlarged elevation and plan views of a portion ofan array of thermal isolation mesa structures in accordance with thisinvention, configured with a mesa-strip conductor between the top of themesa and the integrated circuit substrate.

Thermal isolation mesa structures 40 include a polyimide mesa 41 and amesa-strip conductor 42. The mesa-strip conductor 42 is formed from astrip of conductive material that includes a top portion 42a, a sidewallportion 42b and a contact-pad portion 42c that form a continuousmesa-to-substrate conductor strip. The mesa-to-substrate conductor maybe formed from any suitable conductive material, with a metallicmaterial (such as titanium) being recommended.

The sloped-sidewall contour for mesa 41 facilitates the formation of amesa-to-substrate conductor 42 in the strip configuration. That is, thesloped-sidewall contour makes the transition areas between the mesa-topportion 42a and the mesa-sidewall portion 42b, and between themesa-sidewall portion 42b and the contact-pad portion 42c, less abrupt,and therefore facilitates forming a continuous conductive-strip layer bynormal fabrication processes.

As described in Section 1, selecting an appropriate width and thicknessfor the mesa-strip conductor 42 is a design choice, subject to thedesign goal of minimizing thermal current through the conductor whilemaintaining adequate electrical conductivity. Since any conductivematerial chosen for the mesa-strip conductor 42 will exhibitsignificantly greater thermal conductivity than the polyimide mesa 41,minimizing thermal current through the conductor (i.e., maximizing theconductor's thermal resistance) requires minimizing the cross-sectionalarea-to-length aspect ratio of the conductor strip, taking into accountstructural and electrical requirements. One design approach is to makethe mesa-strip conductor comparable in width to the top of mesa 41, andthen select an appropriate thickness for the conductor to provide theoverall thermal isolation required by the thermal sensor.

A bump-bond metal 45 is disposed over the mesa-top portion 42a of themesa-strip conductor 42. As described in Section 1, the selection of abump-bond metal, and the selection of bump-bonding, are design choicesfor thermal imaging applications of the mesa structure of thisinvention. The inclusion of a bump-bonding metal is exemplary.

3. Mesa Structure (Vertical Sidewall)

FIGS. 4a and 4b show enlarged elevation and plane views of a portion ofan array of thermal isolation mesas with mesa structures havingsubstantially vertical sidewalls, and with a mesa-contour conductor forproviding the mesa-to-substrate conductive path to the integratedcircuit substrate.

A mesa 51 with substantially vertical sidewalls is disposed on theintegrated circuit substrate adjacent contact pad 22. A mesa-contourconductor 52 is formed over the mesa 51, extending over the substratearea adjacent the mesa, including the contact pad.

Mesa-contour conductor 52 includes a mesa-top portion 52a, amesa-sidewall portion 52b and a substrate portion 52c, forming amesa-to-substrate conductive path. The mesa-to-substrate conductor maybe formed of any suitable conductive material, with a metallic material(such as titanium) being recommended.

A mesa-contour configuration for a mesa-to-substrate conductor isrecommended because that configuration is easier to fabricate over amesa structure with vertical sidewalls. That is, if the verticalsidewall configuration for mesa 51 is selected rather than the slopedsidewall configuration shown in FIG. 3a, a strip mesa-to-substrateconductor is more difficult to fabricate because of the abrupttransitions between the mesa-top portion 52a and the sidewall portion52b, and between the mesa-sidewall portion 52b and the substrate portion52c. With current metal deposition processes, forming themesa-to-substrate conductor as a contour layer facilitates reliablycontrolling thickness, uniformity and continuity. The mesa-contourconfiguration for conductor 52 is, thus, a design choice resulting fromconstraints in fabrication processes--with the appropriate selection ofa strip-deposition procedure, a strip configuration for conductor 52could be used for a mesa structure with vertical sidewalls.

As described in Section 1, and as in the case of the mesa-stripconfiguration for a mesa-to-substrate conductor, the selection ofcontour area and thickness for the mesa-contour conductor 52 is a designchoice that depends upon thermal resistance and fabricationconsiderations. That is, the thickness and total area of themesa-contour conductor is selected to achieve a desired amount ofthermal resistance, within practical processing constraints, for theconductive material selected.

From Section 1, the recommended design approach is to configure themesa-contour conductor 52 to exhibit the thermal resistance required bythe thermal sensors. Because the mesa-contour configuration for amesa-to-substrate conductor requires considerably greater area than thestrip configuration described in Section 2, achieving comparable thermalresistance for the two conductor configurations necessarily requiresthat the mesa-contour conductor 52 be considerably thinner than a stripconfiguration. That is, assuming a vertical sidewall mesa comparable insize and thermal resistance to a sloped-sidewall mesa, then themesa-contour conductor 52 should be made comparable in thermalresistance to the mesa-strip conductor (42 in FIG. 3a), therebyrequiring that the mesa-contour conductor be significantly thinner thanthe mesa-strip conductor.

A bump-bond metal 55 is disposed over the mesa-top portion 52a of themesa-contour conductor 52. As described in Section 1, the selection of abump-bond metal, and the selection of bump-bonding, are design choicesfor thermal imaging applications of the mesa structure of thisinvention. The inclusion of a bump-bonding metal is exemplary.

4. Mesa Structure Fabrication

The mesa structures of the present invention, including the exemplarythermal isolation mesas for a thermal imaging system, are fabricatedusing conventional photolithographic techniques. Two separatefabrication methods are described, one using photosensitive polyimideand one using non-photosensitive polyimide. The fabrication method usingphotosensitive polyimide is recommended, because it uses fewer processsteps.

The first step is to apply the polyimide (photosensitive ornon-photosensitive) to an integrated circuit substrate, spinning thesubstrate to flow the polyimide over the substrate to a uniformthickness.

For the recommended fabrication method using photosensitive polyimide,the polyimide is then exposed using a mask that patterns the polyimidelayer. The exposed polyimide is developed using an appropriate solventto remove the non-exposed polyimide, leaving the patterned array ofpolyimide mesas (each adjacent to a respective contact pad on theintegrated circuit substrate). Sidewall configuration is determined byappropriate exposure and development techniques. The assembly is thencured in an appropriate heat-curing process to stabilize and harden thearray of polyimide mesas.

For the alternate fabrication method using non-photosensitive polyimide,the spun polyimide layer is first cured to harden and stabilize thepolyimide. A layer of metal is then applied over the surface, andphotolithographically patterned and etched to create a metal mask. Theunmasked polyimide is etched, and then the metal mask is removed byetching. The remaining polyimide forms the array of mesa structuresadjacent respective contact pads. In this case, sidewall configurationis determined by the organic etch chemistry. This alternate fabricationmethod is less desirable than the recommended fabrication method becauseof the additional metallization steps required.

Once the array of polyimide mesa structures is defined, themesa-to-substrate conductors are formed using conventionalphotolithographic techniques. That is, the mesa-to-substrate conductors,either in the strip configuration of FIG. 3a or the contourconfiguration of FIG. 4a, are formed by either an etch process or a liftprocess using patterned photoresist. The mesa-to-substrate conductorsare formed over the mesa structures, and the adjacent contact pad, to adesired thickness.

The thermal isolation mesas are then complete. Additional fabricationsteps may be employed to deposit bump-bond metals or conductive epoxiesto the top of a mesa (i.e., to the mesa-top portion of amesa-to-substrate conductor) as appropriate. These additionalfabrication steps are accomplished conventionally, with conventionalmaterials the selection of which depends upon the specific applicationfor the mesa structures of this invention.

The thickness of the metal layers used to form the mesa-to-substrateconductors is selected to provide conductors of a predeterminedthickness corresponding to the conductor configuration selected and toachieve a selected conductor thermal resistance, as described inSections 2 and 3.

Additional steps to insert additives into the polyimide material can beused to achieve certain performance criteria. For example, thermalexpansion additives can be added to reduce thermal expansion, andtherefore reduces stresses at the mesa/conductor interface. In addition,foaming additives can be used to reduce thermal conductivity and mass.

Either of the described methods for fabricating mesa structures inaccordance with this invention uses significantly fewer process stepsthan required to fabricate other thermal isolation structures. Forexample, for the via structure described in the Background portion ofthis specification, about 60 process steps are needed to form thethermal isolation structure on the integrated circuit substrate (i.e.,prior to any bump-bonding or other bonding procedures). In contrast, therecommended fabrication method using photosensitive polyimide requiresabout 10 process steps, while the alternate fabrication method usingnon-photosensitive polyimide requires about 25 process steps.

5. Additional Embodiments

The precise structural configuration, and associated fabrication method,for mesa structures in accordance with this invention are significantlydependent upon the application chosen for the mesa structures. Evenwithin a particular application, such as the exemplary thermal imagingsystems, numerous design choices will be routinely implemented by thoseskilled in the art. The basic mesa structure uses polyimide (or asimilar polymer material) to provide mesa structures with a high thermalresistance and an acceptable amount of compliance.

Other applications for mesa structures in accordance with this inventioninclude probe devices and spacers. While a multiprobe device fabricatedusing mesa structures as probes will likely include a mesa-to-substrateconductor, a spacer application probably would not require thatcomponent (or the associated fabrication procedure). In either case,compliance characteristics will compensate for topological variations,and thermal insulation will be provided.

For larger mesa structures, an alternative fabrication method could usescreen printing rather than a photolithographic procedure. A screen maskthat defines the mesa structures would be disposed over a substrate, andthe polyimide forced through the holes in the screen to produce thedesired array of mesa structures.

FIG. 5 illustrates an alternative embodiment in which the metallicconductors (either strip or contour) are obviated by implanting orimpregnating the polyimide (or other polymer) with a suitable dopant toinduce electrical conductivity. Such a procedure would eliminate theconductor forming steps --the mesa structure 61 would merely beconfigured with at least a portion located over a correspondingintegrated circuit contact pad 62, with electrical conductivity from thetop of the mesa 65 to the contact pad being provided by the dopantinduced conductivity. Using a dopant to induce conductivity wouldnecessarily decrease the thermal resistance of the polyimide, leading toa design trade-off between thermal resistance and electricalconductivity.

6. Conclusion

The thermal isolation mesas of this invention use polyimide (or asimilar polymer material) to achieve a high degree of thermal resistancefor the mesa structure. For the exemplary application for thermalimaging systems, the mesa structures are fabricated with amesa-to-substrate conductor (either a strip or contour configuration) toprovide an electrical connection between an array of thermal sensors,and a corresponding array of integrated circuit contact pads, whilethermally isolating the focal plane array from the integrated circuitsubstrate.

In addition to providing a high degree of thermal isolation, thepolyimide mesa structures are sufficiently compliant to accommodate thetopological variations in the mesa structures and the underlyingsubstrate on which they are formed, thereby reducing tolerance andalignment requirements. The polyimide mesa structures can be fabricatedusing either photosensitive or non-photosensitive polyimide; in eithercase, the number of fabrication steps for the mesa structures issignificantly fewer than the fabrication steps required for alternativethermal isolation configurations.

Although the present invention has been described with respect to aspecific, preferred embodiment, various changes and modifications may besuggested to one skilled in the art, and it is intended that the presentinvention encompass such changes and modifications as fall within thescope of the appended claims.

What is claimed is:
 1. A method of fabricating an array of thermallyinsulating mesa structures on a substrate having a substantially planarsurface, comprising the steps of:(a) providing a circuit-containingsubstrate having a substantially planar surface; (b) applying a layer ofthermally insulating polymer material onto said surface of saidcircuit-containing substrate; (c) patterning said layer of thermallyinsulating polymer material to define an array of mesa structures, eachof said mesa structures having a top; (d) providing on said substrate anarray of signal contact pads, each of said contact pads located at saidsurface adjacent and associated with a respective said mesa structure;and (e) for each of said mesa structures, forming a conductor thatprovides a signal path between said top of each of said mesa structuresand the signal contact pad associated therewith.
 2. The fabricationmethod of claim 1, wherein the step of forming a conductor comprises thestep of forming a strip of conductive material from the top of said mesastructure down a sidewall thereof to said associated
 3. The fabricationmethod of claim 1, wherein the step of forming a conductor comprises thestep of forming a contoured layer of conductive material extending fromthe top of said mesa structure down the sidewalls thereof to cover thearea of said circuit substrate adjacent said mesa structure including atleast a portion of the associated contact pad.
 4. The fabrication methodof claim 1, wherein the step of forming a conductor comprises the stepof implanting a conductive dopant in said mesa structure extending fromthe top of said mesa structure to said contact pad, said mesa structurebeing formed with at least a portion in contact with the adjacent signalcontact pad.
 5. The fabrication method of claim 1, wherein said polymermaterial comprises polyimide.
 6. The fabrication method of claim 5,wherein said polyimide material is photosensitive.
 7. The fabricationmethod of claim 6, wherein the step of patterning said polyimide layercomprises the steps of:applying a pattern mask over the polyimide layer,leaving mesa structure areas exposed; irradiating said polyimide layerthrough said pattern mask; developing said polyimide layer by removingunexposed portions and leaving the exposed polymide mesa structures; andcuring said polyimide mesa structures.
 8. The fabrication method ofclaim 5, wherein said polyimide material is non-photosensitive.
 9. Thefabrication method of claim 8, wherein the step of patterning saidpolyimide layer comprises the steps of:curing said polyimide layer;applying a pattern mask over said polyimide layer, leaving mesastructure areas unexposed; removing the exposed portions of saidpolyimide layer, leaving the unexposed polyimide mesa structures; andremoving said pattern mask.